The following relate to the present invention and are hereby incorporated by reference in their entirety: U.S. Pat. No. 5,739,086 Structures Having Enhanced Biaxial Texture and Method of Fabricating Same by Goyal et al., issued Apr. 14, 1998; U.S. Pat. No. 5,741,377 Structures Having Enhanced Biaxial Texture and Method of Fabricating Same by Goyal et al., issued Apr. 21, 1998; U.S. Pat. No. 5,898,020 Structures Having biaxial Texture and Method of Fabricating Same by Goyal et al., issued Apr. 27, 1999; U.S. Pat. No. 5,958,599 Structures Having Enhanced Biaxial Texture by Goyal et al., issued Sep. 28, 1999; U.S. Pat. No. 5,964,966 Method of Forming Biaxially Textured Substrates and Devices Thereon by Goyal et al., issued Oct. 21, 1999; and U.S. Pat. No. 5,968,877 High To YBCO Superconductor Deposited on Biaxially Textured Ni Substrate by Budai et al., issued Oct. 19, 1999.
The present invention relates to electrically conducting and mechanically robust nitride buffer layers which can be deposited epitaxially on biaxially textured surfaces, metals and alloys. The invention also discloses a method to form such epitaxial layers using high rate deposition methods.
Recent emergence of the high-temperature superconducting (HTS) thick-film tape technology is expected to meet the cost, size and performance requirements of superconducting components needed for advanced power applications for the defense and commercial sectors. One of the major potential HTS applications is in the electric power industry.
The YBa2Cu3O7 and related ceramic materials (YBCO) have appropriate intrinsic properties in the liquid nitrogen temperature range. However, their properties are drastically affected by grain boundary misorientations. Hence, in order to enable applications of HTS at high temperature and high fields, it is imperative that a biaxially textured, polycrystalline YBCO tape, or related article, be developed which contains a minimal number of high angle grain boundaries.
One of the industrially scalable processes for producing biaxially textured YBCO conductors is by using Rolling Assisted Biaxially Textured Substrates (RABiTS). As described more fully below, a biaxially textured, flexible metal/alloy substrate is formed by conventional thermomechanical processing followed by epitaxial deposition of buffer layer(s), YBCO grown on such substrate often exhibited Jc""s over 1 MA/cm2 at 77K. To date, the preferred buffer layers for the RABiTS approach have been the combination of CeO2 and YSZ. However, these oxide buffer layers lack important properties, e.g., electrical and thermal conductivity and mechanical toughness. It has been a challenging engineering task to develop a large-scale continuous process for producing thick ( greater than 0.5 xcexcm) crack-and pore-free oxide films. Microcracking in oxide films is commonly observed in thick films due to the brittle nature of the oxide materials. Microcracks in the oxide layer can serve as open paths for oxygen to diffuse and oxidize the underlying metal during subsequent YBCO processing. Finally, the oxide deposition step on the Ni substrates is difficult; high quality films are only obtained by using very low deposition rates. In addition, as with many HTS applications, conductive buffer layers are needed since they would provide electrical coupling of the HTS layer to the underlying metallic tape substrate. This is an important property in order to electrically stabilize the conductor during transient loss of superconductivity in some applications.
Numerous applications of high temperature superconductors, such as transformers, generators and motors require high current carrying, flexible conductors which can sustain magnetic fields above 0.1T. Due to the thermally activated flux flow, the critical current density of most of the highly anisotropic superconducting compounds, such as the Bi-based compounds, rapidly drops at 77K in the presence of an externally applied magnetic field. Moreover, it is not clear if the uniaxially textured, Bi-based wires which typically contain numerous high angle grain boundaries and have approximately ⅔ of their cross-sectional area occupied by silver, will ever reach adequate cost/performance levels for large scale commercial applications. Hence, the development of a viable and low cost processing route based on (Y or Re)Ba2Cu3Ox (YBCO) is of great interest currently and forms a central research thrust in the area of high temperature superconductivity, YBCO compounds have favorable intrinsic properties. Epitaxial YBCO thin films on single-crystal substrates yield critical current densities (Jc""s) in the range of 106-107 A/cm2 at 77K, 0T. YBCO films also have a high irreversibility field of xcx9c6T at 77K.
Conventional ceramic fabrication methods which can be used to make a long, flexible conductor result in materials with weak, if any, macroscopic or microscopic in biaxial texture. In particular, YBCO materials fabricated using conventional techniques invariably contain numerous high angle grain boundaries. High angle grain boundaries act as Josephson coupled weak-links leading to a significant field-dependent suppression of the supercurrent across the boundary. For clean stoichiometric boundaries, the grain boundary critical current density depends primarily on the grain boundary misorientation. The dependence of Jc(gb) on misorientation angle was first determined by Dimos et al. in YBCO for grain boundary types which can be formed in epitaxial films on bicrystal substrates. These include [001] tilt, [100] tilt, and [100] twist boundaries. In each case high angle boundaries were found to be weaklinked. The low Jc observed in randomly oriented polycrystalline HTS fabricated using conventional methods can be understood on the basis that the population of low angle boundaries is small and that frequent high angle boundaries impede long-range current flow. Hence, controlling the grain boundary misorientation distribution towards low angles is key to fabricating high-Jc materials. Practically speaking, this limitation entails the fabrication of biaxially textured superconductors.
Successful fabrication of biaxially textured superconducting wire based on the coated conductor technology, requires optimization of the cost/performance of the HTS conductor. From a superconducting performance standpoint, a long, flexible, single crystal-like wire is required. From a cost and fabrication standpoint, an industrially scalable, low cost process is required. Both of these critical requirements are met by Rolling-assisted-biaxially-textured-substrates (RABiTS). However, in order for cost/performance for a conductor based on this technology to be optimized, further work needs to be done in the area of buffer layer technology. It is now clear that while it is fairly straightforward to fabricate long lengths of biaxially textured metals or alloys, it is quite difficult to deposit high quality buffer layers using low cost processes. Requirements of buffer layers includexe2x80x94it should provide an effective chemical barrier for diffusion of deleterious elements from the metal to the superconductor, provide a good structural transition to the superconductor, have a high degree of crystallinity, excellent epitaxy with the biaxially textured metal template, have good mechanical properties, high electrical and thermal conductivity and should be able to be deposited at high rates.
Buffer layers of the prior art include use of YSZ and CeO2, typically a configuration of CeO2 (0.01 xcexcm)/YSZ (0.5 xcexcm)/CeO2 (0.01 xcexcm). The purpose of the first buffer layer is to provide a good epitaxial oxide layer on the reactive, biaxially textured Ni substrate without the formation of undesirable NiO. CeO2 is special in its ability to very readily form single orientation cube-on-cube epitaxy on cube textured Ni. Deposition of CeO2 using a range of deposition techniques is done using a background of forming gas (4%H2-96%Ar) in the presence of small amounts of water vapor. Under such conditions the formation of NiO is thermodynamically unfavorable while the formation of CeO2 is thermodynamically favorable. The water vapor provides the necessary oxygen to form stoichiometric CeO2. It is not possible to deposit YSZ under such conditions with no evidence of undesirable orientations. In the case of CeO2 one can readily obtain a single orientation, sharp cube texture. Ideally, it would be desired that the CeO2 layer be grown thick such that it also provides a chemical diffusion barrier from Ni, followed by deposition of YBCO. However, when the CeO2 layer is grown greater than 0.2 xcexcm in thickness, it readily forms micro-cracks. Hence a YSZ which does provide an excellent chemical barrier to diffusion of Ni and does not crack when grown thick is deposited on a thin initial template of CeO2. Initial demonstrations at ORNL were using this configuration. However, since there is a significant lattice mismatch between YSZ and YBCO (xcx9c5%), a second 45xc2x0-rotated orientation nucleates at times. In order to avoid the nucleation of this second orientation completely, a thin CeO2 layer is deposited epitaxially on the YSZ layer. This completes the buffer layer structure. YBCO can now be e deposited on the layer which has an excellent lattice match with YBCO (xcx9c0.1%).
The drawbacks of this buffer layer structure are that the deposition of the first CeO2 layer is non-trivial. Strict control of deposition conditions in particular, the O2 partial pressure is required to avoid formation of undesirable NiO (NiO typically Gus nucleates in mixed orientations and is also very brittle). Furthermore, CeO2 can have wide range of oxygen stoichiometry. It is brittle and is not conducting. It will be a challenging engineering task to develop a large-scale continuous process for producing thick ( greater than 0.5 xcexcm) crack-and porosity-free oxide films based on a vapor phase process. For example, in a continuous process involving reactive electron beam evaporation of Ce to form CeO2 , issues relating to the formation of an oxide on the target complicate matters relating to rate of deposition as well as stability of the melt pool. Any change of conditions during deposition are known to have profound affects on the film microstructure. Moreover, any oxidation of the biaxially textured metal, even after the successful deposition of CeO2, can induce undesirable interfacial stresses leading to spallation or further cracking, thus deteriorating the material properties. Microcracks in the oxide buffer layer will adversely affect the epitaxial quality of the growing YBCO film and create weak-links, besides serving as diffusion paths for Ni. Lastly, the surface morphology of the buffer layer is important for subsequent YBCO growth. Ideally, it would be desired to have a buffer layer which tends to be smoother than the Ni substrate it is grown on. In other words, the buffer layer has a sharper rocking curve than the underlying substrate. For CeO2 the rocking curve is almost the same as that for the Ni substrate. All things considered, buffer layer deposition of the prior art is time-consuming and qualitatively deficient.
Macroscopically, biaxially textured YBCO conductors have been formed by epitaxial deposition of YBCO on flexible substrates formed by two techniquesxe2x80x94(a) A flexible, unoriented, polycrystalline metal substrate coated with an oxide buffer film(s) with a forced biaxial texture induced by ion-beam-assisted-deposition (IBAD), where an assisting noble gas beam extracted from an ion source is directed onto the growing film. A similar biaxial texture is observed during oblique vapor deposition on an inclined polycrystalline substrate and (b) A biaxially textured, flexible metal based substrate formed by conventional thermomechanical processing followed by epitaxial deposition of buffer layer(s). This technique is referred to as Rolling Assisted Biaxially Textured Substrates (RABiTS). Using both these techniques Jc""s over 1 MA/cm2 at 77K have been achieved.
The foregoing background information, together with other aspects of the prior art, are disclosed more fully and better understood in light of the following publications:
1. Kormann, G., Bilde, J. H., Sorensen, K., de Reus, R., Anderson R. H, Vace, P., and Fraltoft, T., xe2x80x9cRelation between Critical Current Densities and epitaxy of Y-123 Thin Films on Mgo (100) and SrTiO3 (100),xe2x80x9d J. Appl. Phys., 1992, 71, 3419-3426.
2. Matsuda, H., Soeta, A., Doi, T., Aikhara, and T. Kamo, xe2x80x9cMagnetization and Anisotropy in Single Crystals of Ti-1223 of Tlxe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O System,xe2x80x9d Jpn. J. Appl. Phys., 1992, 31, L1229-31.
3. Dimos, D., Chaudhari, P., Mannhart, X., and F. K. LeGoues, xe2x80x9cOrientation Dependence of Grain Boundary Critical Currents in YBa2Cu3,Ox Bicrystals,xe2x80x9d Phys. Rev. Lett., 1988, 61, 219-222; Dimos, D., Chaudlhari, P., and Mannhart, J., xe2x80x9cSuperconducting Transport Properties of Grain-boundaries in YBa2Cu3O, Bicrystalsxe2x80x9d, Phys. Rev. B, 1990, 41, 4038-4049.
4. Iijima, Y., Tanabe, N., Kohno, O., and Ikeno, Y., xe2x80x9cIn-plane Aligned YBa2Cu3Ox Thin-Films Deposited on Polycrystalline Metallic Substrates,xe2x80x9d Appl. Phys. Lett., 1992, 60, 769-771.
5. Reades, R. P., Burdahl, P., Russo, R. E., and Garrison, S. M., xe2x80x9cLaser Deposition of Biaxially Textured Yttria-stabilized Zirconia Buffer Layers on Polycrystalline Metallic Alloys for High Critical Current Yxe2x80x94Baxe2x80x94Cuxe2x80x94O Thin-films,xe2x80x9d Appl. Phys. Lett., 61, 2231-2233, 1992.
6. Wu, X. D., Foltyn, S. R., Arendt, P. N., Blumenthal, W. R., Campbell, I. H., Cotton, J. D., Coulter, J. Y., Hints, W. L., Matey, M. P., Safar, H. F., and Smith, J. L., xe2x80x9cProperties of YBa2Cu3Ox Thick Films on Flexible Buffered Metallic Substrates,xe2x80x9d App. Phys. Lett., 1995, 67, 2397.
7. Hasegawa, K., Fujino, K., Mukai, H., Konishi, M., Hayashi, K., Sato, K., Honjo, S., Satao, Y., Ishii, H., and Iwata, Y., xe2x80x9cBiaxially Aligned YBCO Film Tapes Fabricated by All Pulsed Laser Depositionxe2x80x9d, Appl. Sppercond., 4, 475-486, 1996.
8. Goyal, A., Norton, D. P., Christen, D. K., Specht, E. D., Paranthaman, M., Kroeger, D. M., D. P., Budai, J. D., He, Q., List, F. A., Feenstra, R., Kerchner, H. R., Lee, D. F., Hatfield, E., Martin, P. M., Mathis, J., and Park, C., xe2x80x9cEpitaxial Superconductors on Rolling-Assisted-Biaxially-Textured-Substrates (RABiTS); xe2x80x9cA Route Towards high Critical Current Density Wire,xe2x80x9d Applied Suppercond., 1996, 69, 403-427.; A. Goyal et al., U.S. Pat. No. 5,739,086 and U.S. Pat. No. 5,741,377.
9. Norton, D. P., Goyal, A., Budai, J. D., Christen, D. K., Kroeger, D. M., Specht, E. D., He, Q., Saffian, B, Paranthaman, M., Klabunde, C. E., Lee, D. F., Sales, B. C., and List, F. A., xe2x80x9cEpitaxial YBa2CU3Ox on Biaxially Textured Nickel (100): An Approach to Superconducting Tapes with High Critical current Density,xe2x80x9d Science, 1996, 274, 755.
10. Goyal, A., Norton, D. P., Kroeger, D. M., Christen, D. K., Paranthaman, M., Specht, E. D., Budai, J. D., He, Q., Saffian, B., List, F. A.; Lett, D. F., Hatfield, E., Martin, P. M., Klabunde, C. E., Mathis, J., and Park, C., xe2x80x9cConductors With Controlled Grain Boundaries: An Approach To The Next Generation, High Temperature Superconducting Wire,xe2x80x9d J. Mater. Res., 1997, 12. 2924-2940.
11. Goyal, A., Norton, D. P., Budai, J. D., Paranthaman, M., Specht, E. D., Kroeger, D. M., Christen, D. K., He, Q., Saffian, B., List, F. A., Leo, D. F., Martin, P. M., Klabunde, C. E., Hatfield, E., and Sikka, V. K., xe2x80x9cHigh Critical Current Density Superconducting Tapes By Epitaxial Deposition of YBa2Cu3Ox Thick Films on Biaxially Textured Metals,xe2x80x9d Appl. Phys. Lett., 1996, 69, 1795.
12. W. D. Sproul, xe2x80x9cPhysics and Chemistry of Protective Coatings,xe2x80x9d Ed. by W. D. Sproul, J. E. Greene, and J. A. Thornton (AIP Conf. Proc. No. 149, 1986, New York), p. 157.
There are a considerable number of problems and deficiencies associated with the prior art relating to the use and deposition of buffer layers on metal/alloy substrates, including those discussed above. There is a demonstrated need for such materials, layers, composites and related articles, together with related methods of preparation.
Accordingly, it is an object of the present invention to provide new and useful conducting and robust, nitride buffer layers on biaxially textured substrates, overcoming those various shortcomings and deficiencies of the prior art, including those described above.
It will be understood by those skilled in the art that one or more aspects of this invention can meet certain objectives, while one or more other aspects can meet certain other objectives. Each objective may not apply equally, in all instances, to every aspect of the present invention. As such, the following objects can be viewed in the alternative with respect to any one aspect of the present invention.
It is another object of the present invention to provide epitaxial nitride layers using a high rate deposition process.
It is yet another object of the present invention to provide epitaxial nitride layers using standard deposition techniques without the use of forming gas during deposition.
It is yet another object of the present invention to provide for the use of nitride layers as templates to grow epitaxial metal/alloy/ceramic and/or device layers.
It is yet another object to provide an alternate buffer layer technology for use in conjunction with the efficient, economical production of superconducting tapes.
Other objects, features and advantages of the present invention will be apparent from the foregoing, in light of the following summary and descriptions, and will be readily apparent to those skilled in the art having knowledge of various buffer layers, composites, articles and their methods of manufacture. Such objects, features, benefits and advantages will be apparent from the above as taken in conjunction with the accompanying examples, tables, data, and all reasonable inferences to be drawn therefrom.
In accordance with one aspect of the present invention, the foregoing and other objects can be achieved by a composite which includes a biaxially textured substrate having thereon a nitride layer.
In accordance with another aspect of the present invention, the foregoing and other objects are achieved by a biaxially textured article which includes a biaxially textured metal or alloy substrate having thereon an epitaxial nitride layer.
In accordance with yet another aspect of the present invention, the foregoing and other objects can be achieved by a biaxially textured article which includes a biaxially textured substrate having thereon an epitaxial nitride layer, the epitaxial nitride layer having thereon an epitaxial layer of an oxide such as, but not limited to, MgO, CeO2, YSZ, LaAlO3, SrTiO3, LaNiO3, Y2O3, RE2O3. (xe2x80x9cRExe2x80x9d is an acronym used for any one of the Rare Earth elements, such as may be present in oxide form, including but not limited to those otherwise listed herein.)
In accordance with still yet another aspect of the present invention, the foregoing and other objects can be achieved by a composite or such article of manufacture which includes a biaxially textured substrate having thereon an epitaxial nitride layer, the epitaxial nitride layer having thereon an epitaxial layer of an oxide such as MgO, CeO2, YSZ, LaAlO3, SrTiO3, LaNiO3, Y2O3, RE2O3; the oxide layer having thereon an additional electromagnetic, epitaxial device layer such as, but not limited to, a superconducting layer.
In accordance with yet a further aspect of the present invention, a method of preparing a biaxially textured article comprising the steps of providing a biaxially textured substrate having a surface and depositing onto the surface, in the presence of nitrogen gas, a layer of a nitride; and further depositing on the nitride layer a layer of an oxide such as, but not limited to, MgO, CeO2, YSZ, LaAlO3, SrTiO3, LaNiO3, Y2O3, RE2O3.
In accordance with still a further aspect of the present invention, a method is provided by way of preparing a biaxially textured composite and/or article comprising the steps of providing a biaxially textured substrate having a surface and depositing onto the surface, in the presence of nitrogen gas, a layer of a nitride; and further depositing on the nitride layer a layer of an oxide such as, but not limited to, MgO, CeO2, YSZ, LaAlO3, SrTiO3, LaNiO3, Y2O3, RE2O3; and further depositing on the oxide layer an additional electromagnetic material layer such as a superconducting layer, en route to an integrated device.
A chemically new type of buffer layer, as can be used with rolling-assisted-biaxially-textured substrates, is disclosed herein. These buffer layers involve the use of mechanically tough and hard compounds which are also conducting. Such buffer layers have high electrical and thermal conductivity and hence will be superior for many electronic or other integrated applications where good electrical and thermal contact between the metal and the integrated device layer is required. Furthermore, a particular high speed deposition process is described, one which achieves epitaxy on a substrate without the use of noble metals or reducing atmosphere. Stoichiometric control allows tailoring of the lattice parameter so as to provide a good buffer between the lattice matched metal and the desired electronic/integrated layer.
As described elsewhere herein, the present invention provides a suitable substrate with a hard, wear-resistant coating which is also thermally and electrically conducting. With regard to thermal conductivity, the layers of this invention provide good contact between an overlaid superconducting layer and the substrate in the event of local loss of superconductivity, thereby providing thermal stabilization. With regard to the corresponding electrical properties, the layers of this invention provide a mechanism for current transfer between the substrate and a superconductor layer, thereby greatly decreasing the problems related to making sufficient electrical contacts to the conductor.
This invention, as evident from this summary, together with the following descriptions and examples, can be utilized with biaxially textured substrates. However, comparable results and benefits are available through use with untextured or randomly textured metal or alloy substrates. In addition to the nitride deposits/layers of this invention, the resulting articles can further include an oxide buffer layer of, but not limited to, MgO, CeO2, YSZ, LaAlO3, SrTiO3, LaNiO3, Y2O3, RE2O3. Alternatively, the untextured or randomly textured substrates can further include such an oxide buffer layer. Regardless, the resulting composite/article can also include an electromagnetic device and/or superconducting layer deposited thereon.
The methods of manufacture associated with this invention are much more time-efficient than those related methods of the prior art. Reactive sputtering of the type described herein, is a relatively rapid-process, with typical rates approaching 0.8 xcexcm/min. Accordingly, the methods of this invention can be utilized to enhance the economics associated with production of long-length superconductors.